Differences in power and performance during sit-to-stand test and its relationships to functional measures in older adults with and without Parkinson ’ s disease

Aims: i) to compare 30-s sit-to-stand (STS) test repetitions and power between older adults with and without Parkinson ’ s disease (PD) and ii) to evaluate the relationship of STS repetitions and power with functional measures in older people with PD. Methods: STS repetitions and power (Alcazar ’ s equation) during the 30-s STS test were assessed in forty-six age-and sex-matched older adults with and without PD. Functional measures included habitual (HGS) and maximum gait speed (MGS), timed-up-and-go (TUG) test and the Mini-Balance Evaluation System Test (Mini-BEST). PD-specific tests were as follows: the motor subscale of the Unified Parkinson ’ s Disease Rating Scale (UPDRS-III), quality of life [Parkinson ’ s Disease Questionnaire (PDQ-39)], perceived freezing of gait (FOG questionnaire), and fear of falling [Falls Efficacy Scale (FES)]. T scores, repeated measures ANOVA and linear regression analyses were used. Results: T scores for older adults with PD were (cid:0) 2.7 ± 4.5 for STS repetitions, (cid:0) 5.2 ± 4.2 for absolute STS power, and (cid:0) 3.1 ± 4.6 for relative STS power compared to older adults without PD. T scores for absolute STS power were lower than T scores for STS repetitions ( p < 0.001) and relative STS power ( p < 0.001). Both absolute and relative STS power and STS repetitions showed similar correlations with functional measures ( r = 0.44 to 0.59; both p < 0.05). Relative STS power ( r = (cid:0) 0.55; p < 0.05) and STS repetitions ( r = (cid:0) 0.47 to (cid:0) 0.55; p < 0.05) but not absolute STS power were correlated to PD-specific tests. Conclusions: STS repetitions and power values estimated through the 30-s STS test were lower in older people with PD than without PD. Overall, STS power measures were similarly associated with functional performance as STS repetitions, indicating these power equations can be implemented when assessing lower extremity function in older people with PD.


Introduction
Parkinson's disease (PD) is the second most common progressive neurodegenerative disorder, affecting about two million Americans, the majority of whom are over 65 years of age (Ou et al., 2021).PD is pathologically characterized by a loss of dopaminergic neurons in the nigrostriatal tract with inclusions composed of α-synuclein, referred to as Lewy bodies, in the neuronal perikarya, which are considered markers of neuronal degeneration (Dickson, 2012).PD is clinically defined as a progressive movement disorder characterized by bradykinesia, resting tremor, rigidity and postural instability.Further, nonmotor symptoms include cognitive and behavioural changes, autonomic nervous system failure, and sensory and sleep disturbances (Radhakrishnan and Goyal, 2018).
Although there are several therapeutic options that can mitigate the symptoms of PD, the progression of this disease leads to impairments that limit performance of activities of daily living (ADL) causing severe disability (Shulman et al., 2008), which translates into a poorer quality of life (Schrag et al., 2000) and an increased risk of mortality (Okunoye et al., 2021).Although the assessment of specific outcomes such as level of rigidity or intensity of tremor is of great importance, functional limitations based on neuromuscular performance have been demonstrated to be the most problematic aspects of a PD patient's disability profile (Bouça-Machado et al., 2018).
One of the most commonly used tests to assess functional performance in older adults with PD is the sit-to-stand (STS) test (Bouça-Machado et al., 2018).The STS test has several versions, the most commonly used being a 5-times STS test (5-rep STS test: time required to complete 5 STS repetitions) and a 30-s STS test (30-s STS test: number of STS repetitions completed in 30 s).These tests have been shown to have excellent test-retest reliability in older people with PD (ICC = 0.94 for 30-s STS and 0.74 for 5-rep STS) (Petersen et al., 2001), and are effective predictors of balance and fall risk in this population (Duncan et al., 2011).
Due to the strong association between power and physical performance (Foldvari et al., 2000;Bean et al., 2003), several equations have been developed in the last few years that allow the computation of power using the number of repetitions performed during the 30-s STS test or the time required to complete the 5-rep STS test (Alcazar et al., 2020;Alcazar et al., 2018;Baltasar-Fernandez et al., 2021a).These equations have been validated against gold standard instruments for assessing power in older populations such as the Nottingham power rig, linear position transducers, force platforms and kinematic analysis.Notably, power values derived from these equations were more closely associated with physical and cognitive function in older people without PD than performance variables such as STS time (Alcazar et al., 2018).Importantly, individuals with PD exhibit diminished muscle power compared to their healthy counterparts, attributable to weakness and bradykinesia in mild to moderate PD stages (Allen et al., 2009).Furthermore, assessing muscle power in older adults with PD is particularly relevant, as reduced muscle power has been associated with slower gait speed and increased risk of falls in this population (Allen et al., 2010).However, evaluating muscle power often requires costly, complex equipment and data analysis, which may prevent its assessment in this population.
Therefore, the use of simple equations to compute muscle power in older adults with PD presents a cost-effective and straightforward alternative.Nevertheless, the application of these equations to estimate muscle power in this population remains unexplored.Consequently, this study aimed to i) compare 30-s STS repetition and power values in older adults with and without PD, and to ii) evaluate the relationship of 30-s STS repetition and power values to functional measures in older people with PD.As a secondary and exploratory objective, this study aimed to analyze possible sex differences in 30-s STS repetition and power values within each group.

Study design and participants
This was a cross-sectional study that included forty-six age-and sexmatched older adults with (n = 23; 9 women) and without PD (n = 23; women) (Table 1).They were recruited from the university population, local clinics and support groups using flyers and an existing call list for the laboratory.
Inclusion criteria were: a) aged 60 years and older; b) physicians' clearance to exercise; c) ability to provide informed consent; d) H&Y stages I-III (Hoehn and Yahr, 1967) (older adults with Parkinson's disease).Individuals were excluded if they: a) lifted weights regularly in the past six months; b) Mini-Mental State Examination score < points; c) myocardial infarction within the past year; d) severe musculoskeletal impairment; f) uncontrolled epilepsy or severe orthostatic hypotension.
All testing was conducted on older adults with PD while they were in an "On" medication state (self-reported).This study was approved by the University Institutional Review Board (IRB) and performed according to the Declaration of Helsinki.All participants were informed of the benefits and risks associated with the testing procedures before signing the informed consent document and proceeding with data collection.

Motor symptoms, quality of life, freezing of gait and fear of falling
The motor subscale of the Unified Parkinson's Disease Rating Scale (UPDRS-III) was used to assess motor symptoms (Fahn, 1987).Quality of life was assessed using the Parkinson's Disease Questionnaire (PDQ-39) (Jenkinson et al., 1997), whereas the perceived freezing of gait (FOG) was evaluated through the FOG questionnaire (Giladi et al., 2000).Finally, fear of falling was measured using the Falls Efficacy Scale (FES) (Tinetti et al., 1990).

Lower-body functional performance
The timed-up-and-go (TUG) test, 5-m walking test for habitual (HGS) and maximum gait speed (MGS), Mini-Balance Evaluation System Test (Mini-BEST) and 30-s STS test were used to assess lower-body functional performance.The TUG test (Podsiadlo and Richardson, 1991) consisted of rising from a chair, walking 3 m, turning 180 • , walking back and sitting down in the chair as fast as possible.TUG velocity was calculated by dividing the distance covered by the recorded time.HGS and MGS were calculated by dividing the 5 m distance covered by the recorded time.Balance was evaluated using the Mini-BEST (King et al., 2012), which contains 14 items divided into 4 domains of dynamic balance.The maximum score is 28 points.Each item is scored from 0 to 2 ("0" indicates the lowest level of functionality and "2" the highest level of functionality).Finally, the 30-s STS test involves recording the number of stands performed in 30 s (Jones et al., 1999).The subjects begin seated on a standardized armless chair (0.45 m) with their arms crossed over their chest.On the command "go", subjects stand up and sit down as many times as possible within 30 s (Jones et al., 1999).
All the tests were assessed to the nearest 0.01 s using a stopwatch.The best time of the two attempts was selected and included in the analysis.

Lower-body power measurements
Lower-body power was calculated using the 30-s STS test and the Alcazar equation (Alcazar et al., 2018).Alcazar's equation uses STS performance (number of repetitions completed in the 30-s STS test), the participant's body mass and height, as well as chair height, to compute mean absolute STS power (W) (Eq.( 1)).In addition, mean absolute STS power was normalized to body mass to obtain mean relative STS power (W•kg − 1 ).

Statistical analysis
Data are presented as mean ± standard deviation (SD) unless otherwise stated.Normality was confirmed by Shapiro-Wilk tests.Differences between older people with and without Parkinson's disease were assessed with independent Student's t-tests.To examine sexdifferences between older adults with and without Parkinson's disease, a two-way ANOVA (sex x group) with Bonferroni's post hoc tests was performed.Cohen's d was also calculated to assess the magnitude and meaningfulness of the differences.To evaluate the magnitude of differences between older adults with PD and those without PD concerning STS repetitions, absolute STS power and relative STS power, sexspecific T scores were calculated using the following formula: T score = (individual value of PD participant -mean of non-PD group) / (SD of non-PD group / √sample size of non-PD group).Then a two-way (sex x test) repeated measures ANOVA with Bonferroni's post hoc tests was used to assess whether the T scores of STS repetitions, absolute STS power and relative STS power differed between sexes and between each other among older adults with PD.Linear regression analyses were used to assess the correlation between STS repetitions and power values with functional outcomes.In addition, differences in r values (β coefficients) were assessed by comparison of 95 % intervals to examine if the relationships of STS repetition and power values with functional measures were statistically different.Statistical analyses were performed using SPSS v25 (SPSS Inc., Chicago, Illinois) and the level of significance was set at α = 0.05.
The differences in mean values by sex for STS repetitions, STS power, and relative STS power are presented in Table 2.A significant group effect was observed for STS repetitions (p = 0.003), absolute STS power (p < 0.001), and relative STS power (p < 0.001).A significant sex effect was observed in absolute (p < 0.001) and relative STS power (p = 0.017) but not in STS repetitions (p = 0.252).Older women with PD showed lower mean values in STS repetitions (p = 0.044), absolute STS (p = 0.001) power, and relative STS power (p = 0.004) compared to men with PD.Men without PD exhibited higher absolute STS power than women without PD (p = 0.040), but similar STS repetitions (p = 0.668) and relative STS power (p = 0.612).Women without PD showed higher mean values in STS repetitions (p = 0.003), absolute STS power (p = 0.001), and relative STS power (p = 0.001) compared to those with PD.Men without PD showed higher mean absolute STS power compared to those with PD (p = 0.005), but similar STS performance (p = 0.283) and relative STS power (p = 0.161).

Comparison of T scores for STS repetitions, absolute STS power, and relative STS power between men and women with PD
T scores for older adults with PD (men and women combined) were − 2.7 ± 4.5 for STS repetitions, − 5.2 ± 4.2 for absolute STS power, and − 3.1 ± 4.6 for relative STS power.T scores for absolute STS power were significantly lower than T scores for STS repetitions (p < 0.001) and relative STS power (p < 0.002) in older adults with PD.Specifically, older men with PD showed STS repetitions of − 1.9 ± 5.2, absolute STS power of − 3.6 ± 3.9, and relative STS power of − 2.4 ± 5.5 standard deviations below older men without PD, while older women with PD showed STS repetitions of − 4.0 ± 3.0, absolute STS power of − 7.7 ± 3.3, and relative STS power of − 4.3 ± 2.5 standard deviations below older women without PD (Fig. 1).The T scores for STS repetitions and relative STS power were similar between men and women (p = 0.282 and p = 0.343, respectively), but T scores for absolute STS power were significantly lower in women compared to men (p = 0.018).T scores for absolute STS power differed significantly from those for STS repetitions p = 0.045) but did not differ from those for relative STS power (p = 0.308) among men.T scores for absolute STS power differed significantly from those for STS repetitions (p < 0.001) and relative STS power (p < 0.001) among women.

Association of STS performance and power with functional measures in older adults with PD
Table 3 presents correlations between STS repetitions and power variable and measures of mobility and balance in the participants with PD.As can be seen from the table, strong and similar correlations were seen between HGS and MGS with all STS measures (r = 0.52 to 0.59; p < 0.05) while only STS repetitions and relative STS power were moderately and similarly correlated to TUG velocity (r = 0.44 and r = 0.46, respectively; both p < 0.05).No correlation was observed between any STS variable with the Mini-Best test.Correlations between STS variables and variables directly associated with PD are presented in Table 4. Strong and similar correlations were found between FES and STS repetitions (r = − 0.57; p = 0.005) and relative STS power (r = − 0.55; p = 0.007).Furthermore, a moderate correlation was seen between the UPDRS mobility score and STS repetitions (r = − 0.45; p = 0.030).

Discussion
This study showed that older adults with PD exhibit STS repetitions, PD: Parkinson's disease; STS: sit-to-stand; p values show mean differences between groups.Bold values indicate significant differences compared to men within the same group (p < 0.05).* Significantly different compared to the group of older people with PD within the same sex (p < 0.05).

Fig. 1.
T scores among older men and women with PD for STS repetitions, absolute STS power, and relative STS power.Note: * Indicates significant differences (p < 0.05) within the same variable compared to older men with PD. # Indicates significant differences (p < 0.05) compared to absolute STS power within the same sex.

Table 3
Correlations between sit-to-stand and lower-body functional performance measures in older adults with Parkinson's disease (n = 23).absolute STS power, and relative STS power averaging between − 2.7 and − 5.2 standard deviations below older adults without PD.In older men with PD, these differences compared to the group without PD range from − 1.9 to − 3.6 standard deviations.In contrast, for older women with PD, these differences are even greater, ranging from − 4.0 to − 7.7 standard deviations compared to the group of women without PD.Furthermore, the relationship between relative STS power, functional performance variables and specific-PD tests was similar to the relationship observed with STS repetitions, and both demonstrated stronger correlations than absolute STS power.Despite the widespread use of the STS test as a functional assessment tool in older people with PD (Bouça-Machado et al., 2018), no research has investigated the potential for using STS power rather than time or repetitions with this population.To our knowledge, there is only one study that has evaluated power in the STS test in older adults with PD, and it used a 6-axis motion sensor placed below the posterior superior iliac crests (Serra-Añó et al., 2020).These researchers found that absolute STS power estimated through the sensor was similar in subjects with and without PD (206.4 vs 210.8 W; p = 0.80).Although the absolute STS power values of people with PD were similar to those obtained in our study using the Alcazar eq. (206.4 vs 199.7 W,respectively), our study revealed significant differences between participants with and without PD (199.7 ± 92.2 vs 296.7 ± 77.0 W; p < 0.05).It is important to note that STS muscle power values estimated using the Alcazar's equation have not yet been validated in older adults with Parkinson's disease.This population exhibits reduced lower limb muscle strength, which necessitates greater trunk flexion to rise from a chair, resulting in increased time to rise and sit down compared to healthy controls (van Lummel and Evers, 2018).These differences could potentially affect the muscle power calculations estimated through the Alcazar's equation.However, a recent study evaluating the STS test with a triaxial accelerometer and gyroscope sensor in older adults with mobility limitations showed that, although these individuals spent more time on standing up from the chair and to sit down or exhibited greater trunk flexion to perform the task, the percentage of time spent rising was the same as the time spent sitting down (Meulemans et al., 2023).The Alcazar's equation assumes that the time spent in the ascent phase is the same as that in the descent phase of STS test, hence, the muscle power estimated through the Alcazar's equation was not different from the muscle power assessed with the triaxial accelerometer and gyroscope sensor in older adults with mobility limitations (Meulemans et al., 2023).This suggests that the power values for older adults with PD in our study may be valid; however, studies validating the implementation of the Alcazar's equation in this population are needed.
Furthermore, our results indicate that the difference in absolute STS power between subjects with and without PD was greater than the differences in STS repetitions completed in the 30-s STS test (− 5.2 SD vs − 2.7 SD, respectively).It is important to note potential biases in the T scores of absolute STS power values among older adults with PD due to significant body mass differences between groups.This arises from the fact that individuals with higher body weight can generate greater STS power at the same number of STS repetitions (assuming similar height).However, it is crucial to highlight that if individuals with and without PD had similar body masses, older adults without PD would likely perform more repetitions in the 30-s STS test, thereby widening the existing differences not only in STS repetitions but also in STS power.For this reason, when normalizing by body mass (i.e., relative STS power), these differences are attenuated, although the T scores still indicate larger values for relative STS power compared to STS repetitions.This disparity between STS repetitions or time and muscle power between older adults with and without PD has also been observed in previous studies.Pääsuke et al. (2004) (Pääsuke et al., 2004) observed that, in patients with PD, chair rise time was 24 % longer and rate of force development while rising from a chair was 30 % lower than in ageand sex-matched participants without PD.Besides, Allen et al. (2009) (Allen et al., 2009) demonstrated that the maximum lower-limb power of older adults with PD was approximately 22 % less than in those without PD.On the other hand, if we take into account the normative values published in older people without PD of relative STS power (Alcazar et al., 2021), we find that on average, our older men with PD (3.2 W•kg − 1 ) had relative STS power values below the 50th percentile (3.6 W•kg − 1 ) while our older women with PD (2.0 W•kg − 1 ) had relative STS power values below the 25th percentile of the general older population (2.4 W•kg − 1 ), which supports the differences found in our research between older people with and without PD.Specifically, women (77.8 %) and 7 men (50 %) exhibited relative STS power values below the 25th percentile, indicating markedly low levels of muscle power compared to older adults without PD.Additionally, 4 men (28.6 %) showed relative STS power values below the 50th percentile, while women (22.2 %) and 1 man (7.1 %) exhibited power values below the 75th percentile.Furthermore, on average, older men without PD (3.7 W•kg − 1 ) exhibited power values above the 50th percentile of the general population (3.6 W•kg − 1 ), whereas older women without PD (3.5 W•kg − 1 ) had power values above the 75th percentile (3.4 W•kg − 1 ), demonstrating that both older men and women without PD exhibited optimal relative STS power values.The fact that women in our group exhibit higher power values relative to their sex than men is reflected not only in the T scores, with the group of women with PD showing larger T scores than the group of men with PD, but also in the comparisons of mean values, with women without PD showing lower absolute power values than men without PD, but similar values in STS repetitions and relative STS power.However, when comparing men and women with Parkinson's, men exhibited higher values in STS repetitions, absolute STS power, and relative STS power, consistent with cohort studies (Alcazar et al., 2021).This may indicate that, regardless of Parkinson's, men are capable of producing more power than women.However, these results are purely exploratory and should be interpreted with caution due to the small sample size in our study.
Regarding the clinical relevance of 30-s STS test, it is a fundamental task associated with many activities of daily living, and the efficiency with which it is performed indicates an individual's level of functional independence.In this regard, it has been reported that 80 % of older people with PD have difficulty getting out of a chair (Brod et al., 1998), which, in older people without PD, is directly related to a higher risk of all-cause mortality compared to those participants who are able to get out of a chair with ease (Losa- Reyna et al., 2021).In our study, all participants were able to perform at least 3 STS repetitions on the 30-s STS test, however, if we take into account the cut-off points for determining low relative STS power in older adults without PD (2.1 and 2.6 W•kg − 1 for women and men, respectively) (Alcazar et al., 2021), a total of 11 (47.8 %) older adults with PD and 1 (4.3 %) older adult without PD had low relative STS power.Having low relative STS power has been shown to increase the risk of frailty, limitations in performing activities of daily living and low gait speed in older people without PD (Baltasar-Fernandez et al., 2021b;Bahat et al., 2020), however, the impact of having low relative STS power in older adults with PD is still unknown and should be investigated in future research.Furthermore, it is important to highlight that this is the first study to demonstrate the relationship between absolute and relative STS power and functional performance variables in older people with PD.STS repetitions, as well as absolute and relative STS power, were associated with both HGS and MGS, while only STS repetitions and relative STS power were associated with TUG velocity and FES.Therefore, both relative STS power and STS repetitions have shown similar correlations with functional performance and PD-specific tests in our study.However, our study is the first to explore the predictive ability of relative STS power versus STS repetitions in a small sample of older people with PD, and the results obtained should be interpreted with caution.We believe that future studies comparing the predictive capacity of relative STS power versus STS repetitions should be conducted, as previous studies with larger sample sizes of older adults without PD have demonstrated that relative STS power is better associated with functional performance than STS repetitions (Alcazar et al., 2020;Alcazar et al., 2018).These results are clinically relevant for older people with PD since gait speed and TUG velocity have been demonstrated to be early predictors of mortality in PD. (Bäckström et al., 2018) Moreover, FES has been associated with increased severity of PD, cognitive function and FOG, demonstrating a close relationship with the number of self-reported falls (Thomas et al., 2010).Despite what is known about power as a predictor of physical and cognitive function in older people without PD (Bean et al., 2003;Alcazar et al., 2020;Alcazar et al., 2018), there are no investigations that have evaluated the strength of such associations in PD.Our research, demonstrating the ability of relative STS power to predict functional performance, holds significant clinical relevance for older adults with PD.

Study limitations
There were three limitations associated with this research.Firstly, the Alcazar's equation has not been validated in older adults with PD, thus, future studies aimed at validating the equation in this population are needed.Secondly, the small sample size limited our ability to obtain clear results regarding the effects of sex and Parkinson's disease on 30-STS repetitions and power.Future research with a larger sample, which includes both older and middle-aged adults with PD across H&Y stages I-V, is necessary to provide more robust insights into the clinical relevance and criterion validity of the 30-s STS test, as well as to explore potential sex differences more comprehensively.Finally, the cross-sectional design did not allow us to establish cause-effect relationships; therefore, future longitudinal studies are required.

Conclusion
STS repetition and power values were lower in older adults with PD compared to those without PD, with women exhibiting notably larger differences, primarily attributed to the superior sex-specific power of the non-PD group.Overall, relative STS power and STS repetitions were associated similarly and with more functional performance variables and specific-PD tests than absolute STS power.Therefore, the Alcazar equation could be employed to assess muscle power in older adults with PD, though further studies are needed to examine the validity and clinical relevance of this test within this population.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Table 1
Baseline characteristics of the study participants.

Table 2
Comparison of the mean values of STS repetitions, absolute STS power, and relative STS power in men and women with and without Parkinson's disease.

Table 4
Correlations between sit-to-stand variables and Parkinson's disease specific tests (n = 23).